Citation: Enebe, N.L.; Chigor, C.B.; Obileke, K.; Lawal, M.S.; Enebe, M.C. Biogas and Syngas Production from Sewage Sludge: A Sustainable Source of Energy Generation. Methane 2023, 2, 192–217. https://doi.org/10.3390/ methane2020014 Academic Editor: Patrick Da Costa Received: 19 March 2023 Revised: 14 April 2023 Accepted: 27 April 2023 Published: 29 April 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Review Biogas and Syngas Production from Sewage Sludge: A Sustainable Source of Energy Generation Nwabunwanne Lilian Enebe 1, *, Chinyere Blessing Chigor 2 , KeChrist Obileke 3 , Mohammed Shariff Lawal 4 and Matthew Chekwube Enebe 5 1 Department of Physics with Electronics, Air Force Institute of Technology, Nigerian Air Force Base, Rafin Kura, Kaduna 800283, Nigeria 2 Department of Plant Science, University of Nigeria, Nsukka 410001, Nigeria 3 Department of Physics, University of Fort Hare, Private Bag X1314, Alice 5700, South Africa 4 Department of Mechanical Engineering, Air Force Institute of Technology, Nigerian Air Force Base, Rafin Kura, Kaduna 800283, Nigeria 5 Centre for Mineral Biogeochemistry, University of the Free State, Private Bag 339, Bloemfontein 9301, South Africa * Correspondence: nlnwokolo@gmail.com; Tel.: +234-706-553-2574 Abstract: Sewage sludge to energy conversion is a sustainable waste management technique and a means of militating against the environmental concerns associated with its disposal. Amongst the various conversion technologies, anaerobic digestion and gasification have been identified as the two most promising. Therefore, this study is focused on a detailed evaluation of the anaerobic digestion and gasification of sewage sludge for energy production. Moreover, the key challenges hindering both technologies are discussed, as well as the practical measures for addressing them. The applicable pretreatment measures for efficient transformation into valuable energy vectors were further evaluated. Specifically, the study evaluated various properties of sewage sludge in relation to gasification and anaerobic digestion. The findings showed that a high ash content in sewage sludge results in sintering and agglomeration, while a high moisture content promotes tar formation, which has been identified as one of the key limitations of sewage sludge gasification. More importantly, the application of pretreatment has been shown to have some beneficial features in promoting organic matter decomposition/degradation, thereby enhancing biogas as well as syngas production. However, this has additional energy requirements and operational costs, particularly for thermal and mechanical methods. Keywords: sewage sludge; anaerobic digestion; gasification; biogas; syngas; energy 1. Introduction Sewage sludge (SS), as observed, is a by-product of wastewater treatment process. Its treatment and management incur about 50% of the total operating costs in wastewater treatment plants (WWTPs) [1]. In terms of generation capacity, approximately 10 million tons of sewage sludge are produced in Europe, 8 million in the United States, and 4 million tons in China annually [2]. The generated sewage sludge poses a threat to the environment and humans alike due to the presence of heavy metal pollutants, a high portion of organic, and toxin contents [3–5]. Despite it being laden with pollutants, the sustainable usage of this growing quantity of produced sewage sludge can make it a valuable resource. Upon processing, sewage sludge can serve as a feedstock or substrate for energy generation [6]. Energy recovery from sewage sludge has received increasing attention in recent years. This could be attributed to the dwindling nature of conventional energy resources (fossil fuel) and the belief that sludge-derived energy, being a renewable source, can assist in achieving a circular green economy [7]. Renewable energy resources such as biomass are considered the most regenerative, renewable, and available raw material for producing different forms Methane 2023, 2, 192–217. https://doi.org/10.3390/methane2020014 https://www.mdpi.com/journal/methane